1. Field of the Invention
The present general inventive concept relates to a switching mode power supply (SMPS) that includes a surge protection circuit for preventing a malfunction of the SMPS.
2. Description of the Related Art
A switching mode power supply (SMPS) converts an alternating current (AC) power supply into a direct current (DC) power supply that is required by various electronic apparatuses such as computers or communication apparatuses. The SMPS has merits such as small size, high efficiency, light weight, and the like compared to a linear type power supply and thus is widely used. In particular, the SMPS is contained in an image forming apparatus such as a printer, a scanner, a fax machine, or a multifunctional printer and converts an AC power supply provided from the outside into a DC power supply to supply the DC power supply to components such as a main controller and the like in the image forming apparatus.
In the SMPS, a primary circuit of a transformer receives an AC power supply and rectifies and smoothes the AC power supply, and then applies a rectified and smoothed voltage to a primary coil of the transformer. Then, a voltage that is induced in a secondary coil of the transformer passes through a secondary circuit of the transformer, and a DC voltage that is output through the secondary circuit is applied to a load. At this time, it is necessary to adjust the output of the secondary circuit, which is applied to the load. This is possible by applying a control signal to a switching device connected to the primary circuit of the transformer by using a pulse width modulation (PWM) controller included in the SMPS to adjust a duty ratio of a current flowing through the primary circuit of the transformer.
The PWM controller receives the output of the secondary circuit of the transformer and may perform a protection function by stopping, rebooting, or resetting the SMPS when the output voltage is in an abnormal state, i.e., an overvoltage or excessive current state. In detail, the output voltage of the secondary circuit of the transformer is fed back to a protection pin of the PWM controller, and the PWM controller senses the abnormal state when a voltage of the protection pin exceeds a threshold voltage and then stops, reboots, or resets the SMPS.